Team Germany punched their ticket to Sunday's deciding final; the Spanish were impressive in front of their home crowd in Barcelona.
Vanessa Mannix (CAN) finished second with Balzane du Mazes, while Brennan's fiancée Lauren Crooks (USA) finished third.
Belgium, Brazil, Canada, France, Great Britain, Germany, Ireland, Italy, Netherlands, Norway, Spain, Sweden, Switzerland, USA and Uzbekistan will compete.
A previous series winner, she reclaimed the title with Havana, a horse she has developed herself since the mare was four years old.
The Irishman kicked off the North American League with a convincing victory aboard Vital Chance de la Roque, edging Canada's Vanessa Mannix.
The pair lead wire-to-wire in the $15,000 CSI4*-W 1.45m at Thunderbird Show Park's Harvest Celebration over Justin Prather and EZ Ace.
The prolific winner places one-two in Friday's $75,000 CSI4*-W during the Harvest Celebration; his student Vanessa Mannix (CAN) claims 3rd.
The Harvest Celebration CSI4*-W at Thunderbird Show Park in Langley, BC, is underway and will feature the $150,000 World Cup™ Vancouver.
Hayes qualified two horses for the jump-off at Wesley Clover Parks over a track set by Canadian course designer Pierre Burch.
Deslauriers, Asselin, and Coyle take the big classes in the International Ring to close out the September Series.